1. Field of the Invention
The present invention relates to an organic EL drive circuit and an organic EL display device using the same and, in particular, the present invention relates to an improvement of an organic EL drive circuit for current-driving a column line (anode side drive line) of each of organic EL elements of an organic EL panel by supplying current, which corresponds to an input digital value and is generated by a D/A converter circuit utilizing a current mirror circuit, to each of terminal pins of the organic EL panel, such that a peak current for driving the organic EL panel can be easily generated by the drive circuit and an area of the drive circuit can be reduced and an organic EL display device using the same organic EL drive circuit.
2. Description of the Prior Art
It has been known that an organic EL display device, which realizes a high luminance display by light generated by itself, is suitable for a display on a small display screen and the organic EL display device has been attracting public attention as the next generation display device to be mounted on a portable telephone set, a DVD player or a PDA (Personal Digital Assistants) such as a portable terminal device, etc.
Known problems of the organic EL display device are that, when it is driven by voltage as in a liquid crystal display device, luminance variation thereof becomes substantial and that, since there is difference in sensitivity between R (red), G (green) and B (blue), a control of luminance of a color display becomes difficult.
In view of these problems, an organic EL display device using current drive circuits has been proposed recently. For example, JPH10-112391A discloses a technique with which the luminance variation problem is solved by employing a current drive system.
An organic EL display panel of an organic EL display device for a portable telephone set, having 396 (132 3) terminal pins for column lines and 162 terminal pins for row lines has been proposed. However, there is a tendency that the number of column lines as well as row lines is further increased.
An output stage of a current drive circuit of such organic EL display panel of the active matrix type or the simple matrix type includes a current source drive circuit, such as an output circuit constructed with a current mirror circuit for each of the terminal pins. A drive stage thereof includes a parallel-driven type current mirror circuit (reference current distribution circuit) having a plurality of output side transistors for each of the terminal pins as disclosed in JP2002-82662 (domestic priority application claiming priorities of JP2001-86967 and JP2001-396219) corresponding to U.S. patent application Ser. No. 10,102,671. In the disclosed drive stage, a plurality of mirror currents are generated correspondingly to the respective terminal pins by branching a reference current generated by the parallel-driven type current mirror circuit to thereby drive the output circuits. Alternatively, the mirror currents distributed to the respective terminal pins are amplified by respective k-time current amplifier circuits, where k is an integer not smaller than 2, and the output circuits are driven with the amplified currents. The drive stage including the k-time amplifier circuits is disclosed in JP2002-33719, in which D/A converter circuits are provided correspondingly to the respective terminal pins. In the disclosed circuit construction, the D/A converter circuit converts display data corresponding to the column side terminal pins into analog data to generate a column side drive currents simultaneously.
In this disclosed construction, a peak current is generated for initially charging an organic EL element having capacitive load characteristics to drive the organic EL element. The peak current may be generated before the drive stage as a reference current, after a D/A converter circuit as described in JP2002-33719 or in a current output stage.
FIG. 5 shows a typical example of the peak current generator circuit for an organic EL display element of an organic EL display panel, which generates the peak current in the current output stage and is disclosed in JPH11-45071A. Further, FIG. 6 shows another example, which is disclosed in JP2002-33719 and in which the peak current generator circuit is provided after the D/A converter circuit.
The example shown in FIG. 5 in which the peak current generator circuit is provided in the current output stage will be described first. In the current drive circuit shown in FIG. 5, a pulse generator circuit 5 generates a pulse signal 6 synchronized with a drive pulse and the pulse signal 6 is supplied to a switching element 8 of an initial charging circuit 7b provided in parallel to a load resistor of a constant current source (current mirror output circuit) 7a of a drive circuit 7. Upon this, the switching element 8 is turned ON and a current flows to an organic EL element 4 through the switching element 8 and a switching transistor 7c, which is simultaneously turned ON by the drive pulse, so that the organic EL element 4 is driven. As a result, a large current flows for a constant time from a start time of the driving, which is determined by an ON resistance of the switching element 8 and a junction capacitance of the organic EL element 4. Therefore, in the initial drive stage, the organic EL element 4 is charged rapidly, so that a luminance of the organic EL element 4 is improved and luminance variation thereof is prevented.
The peak current generator circuit shown in FIG. 6 includes a column driver 1 of an organic EL drive circuit, a D/A converter circuit 2 and a current mirror type current output circuit 3.
The current mirror type current output circuit 3 includes a drive stage current mirror circuit 3a and an output stage current mirror circuit 3b. 
The drive stage current mirror circuit 3a is a peak current generator circuit and includes diode-connected PNP input side transistor Qs and output side transistor Qt. Emitters of these transistors are connected to an input terminal 3c of the output stage current mirror circuit 3b through a P channel MOS FET Trs and an N channel MOS FET Trt, respectively.
A collector of the input side transistor Qs is connected to an output terminal 2b of the D/A converter circuit 2 and a collector of the output side transistor Qt is grounded. An emitter area ratio of the transistor Qs to the transistor Qt is 1:x. Assuming that an output current of the D/A converter circuit 2 is Ia, a drive current generated at the input terminal 3c becomes (x+1) Ia. Therefore, the current mirror circuit 3a generates drive current (1+x) Ia when the transistor Trt is in ON state. The transistor Trs is a load transistor provided correspondingly to the transistor Trt and has a gate connected to GND. The transistor Trs is provided to balance a drive line. Incidentally, the transistor Trt is turned ON for a constant time in the initial stage of driving by a control signal CONT.
The current mirror circuit 3a drives a PNP input side transistor Qx of the output stage current mirror circuit 3b through PNP current mirror transistors Qu and Qw, which are provided for base current correction. As a result, current (1+x) Ia flows through the input side transistor Qx for a constant time during which the transistor Trt is turned ON to perform a peak current drive of the organic EL element. Thereafter, the drive current Ta is outputted as a normal drive current. The current (1+x) Ia and the current Ta are amplified to N times by a PNP type output side transistor Qy of the output stage current mirror circuit 3b and outputted to one (9) of the column side terminal pins of an organic EL panel.
Incidentally, an emitter area ratio of the transistor Qx to the transistor Qy in the output stage current mirror circuit 3b is 1:N and emitters of these transistors are connected to not a power source line +VDD but a power source line +Vcc having a voltage higher than that of the power source line +VDD, that is, in a range from +15V to +20V, and a collector of the output side transistor Qy is connected to the column side terminal pin 9.
Therefore, it is possible to supply the drive current N (1+x) Ia to the column side terminal pin 9 when the peak current drive is performed. Consequently, in the initial stage of the current drive, the organic EL element 4 having the capacitive load characteristics is charged rapidly by the peak current and driven thereby.
The D/A converter circuit 2 includes a diode-connected input side NPN type bipolar transistor Qa and a current I from a constant current source 14a is supplied to a collector of the transistor Qa through an input terminal 2a of the D/A converter circuit 2. The D/A converter circuit 2 further includes output side NPN bipolar transistors Qb to Qn-1, which are connected to the transistor Qa in current mirror relation and N channel MOS FET Trb to Trn-1 connected between emitters of the output side transistors Qb to Qn-1 and ground as switch circuits. Gates of the transistors Trb to Trn-1 are connected to respective input terminals D0 to Dn-1.
Collectors of the output side transistors Qb to Qn-1 are connected to an output terminal 2b and emitter area ratios of the transistors Qb to Qn-1 with respect to an emitter area of the transistor Qa correspond to weights 1, 2, 4, n of respective columns. An emitter of the input side transistor Qa is grounded through a series circuit of a resistor Ra and an N channel MOS FET Tra having a gate connected to the power source line +VDD.
The D/A converter circuit 2 receives at the input terminals D0 to Dn-1 thereof digital display data corresponding to display luminance, which may vary time to time, from a processor such as a CPU or an MPU, etc., and generates at the output terminal 2b analog current values corresponding to the input data (display data). It should be noted that the output circuit of the reference current distribution circuit for one of terminal pins of the drive stage is shown in FIG. 6 as the constant current source 14a. Further, a transistor Trr and a transistor Qr constitute a base current supply circuit for supplying a base current to the current-mirror connected common base line and the transistor Qr has an emitter grounded through a series circuit of a resistor Rr and an N channel MOS FET Trra and a gate connected to the power source line +VDD.
There is a recent tendency that the number of drive pins is increasing due to increase of resolution. Since the peak current generator circuit and the D/A converter circuit are provided correspondingly to each of terminal pins for current driving the organic EL elements, the size of integrated circuit is increasing. Therefore, in order to reduce power consumption and reduce the area occupied by the integrated circuit, which is increased with increase of the number of drive pins, it is important to reduce the size of these circuits.
An object of the present invention is to provide an organic EL drive circuit capable of easily generating a peak current for current driving an organic EL element and of reducing an area occupied by the drive circuit and an organic EL display device using the same organic EL drive circuit.
In order to achieve the above object, a first aspect of the present invention resides in an organic EL drive circuit including a current mirror circuit, which, in response to a predetermined current supplied to an input side transistor portion thereof, generates a predetermined current to be supplied to a terminal pin of an organic EL panel in an output side transistor portion thereof or a current on which the predetermined current is obtained, is featured by that the input side transistor portion includes a plurality of parallel-connected input side transistors and a control circuit for controlling an output current of the output side transistor such that the output current is changed from a peak current to a steady current by reducing a drive current for one of the input side transistors with respect to the current mirror circuit by generating the peak current in the output side transistor portion by current-driving one of the input side transistors and branching the predetermined current to the other input side parallel transistors provided in parallel to the one input side transistor current-driven by the predetermined current.
According to a second aspect of the present invention, in the organic EL drive circuit of the first aspect, the output side transistor portion of the current mirror circuit includes a plurality of output side transistors and a D/A converter circuit is constructed with the plurality of the output side transistors and generates a total value of currents flowing through the output side transistors at its output terminals by making each of the output side transistors correspondent to bit column position of an input data to be D/A converted and selectively operating the output side transistors correspondingly to the input data. A switch circuit is provided in at least one of the input side transistors of the current mirror circuit and a constant current source for generating the predetermined current is provided. The organic EL drive circuit generates a converted analog current having the peak by reducing a drive current for one of the input side transistors of the current mirror circuit by supplying a current from the constant current source to one of the input side transistors to drive the one input side transistor and turning the switch circuit ON at a predetermined time from the drive start time to branch the current from the constant current source through the switch circuit.
According to a third aspect of the present invention, in the organic EL drive circuit of the second aspect, the current mirror circuit includes two input side transistors having operating current ratio of 1:N where N greater than 1, wherein one of the input side transistors having operating current ratio of 1 is supplied with current from the constant current source and supplies a branch current to the other input side transistor corresponding to the operating current ratio of N by turning the switch circuit ON.
As mentioned above, according to the present invention, a plurality of parallel-connected input side transistors of the current mirror circuit are provided and the input side drive current is controlled by inserting the switch circuit in series with one of the input side transistors. A current corresponding to the peak current of the output side transistor is generated by the input side transistor, which is driven first, and the drive current of each of the input side transistors of the current mirror circuit is reduced by branching the drive current to one of the input side transistors by turning the switch circuit ON after a predetermined time from a drive start time by driving one of said input side transistors with the predetermined current, or from the generation of the current of the output side transistor portion or from a drive start time of an organic EL element. Therefore, a large drive current flows at the start time so that a current corresponding to the peak current is obtained by the output side transistor of the current mirror circuit and, after the predetermined time therefrom, the drive current smaller than the initial drive current flows to make the output current of the output side transistor becomes steady current, resulting in that the current having the peak is generated in the output side transistor.
Therefore, the insertion of a resistor in the output stage circuit and the switch circuit for short-circuiting the resistor (corresponding to the switching element 8 shown in FIG. 5), which are necessary in the conventional technology, become unnecessary. Further, the conventional drive current source (corresponding to the drive stage current mirror circuit 3a shown in FIG. 6) dedicated to the peak current generation for adding the peak current becomes unnecessary. Therefore, according to the present invention, the circuit construction of the organic EL panel becomes simple.
As a result, it is easy to generate a drive current having a peak necessary to initially driving the organic EL element and to reduce the area occupied by the drive circuit.